If you use a firewire-driven printer or plotter device, this is the only issue that I have heard people have real issues with.
The ONLY thing that we had problems with when running Flexi 8.1 on a Windows 7 computer system was it would not recognize a firewire output port option to do digital printing to our Mimaki JV3 printer. Just as Sightline post above, there are a lot of people out on the forums that have tried and have almost unanimously succeeded in installing and running Flexi 8.1 in Windows Vista or Windows 7 32-bit or 64-bit.
It is, however, not SUPPORTED meaning that if you call in to SAi or any of their resellers the only help they will give you is, 'Give us more $$$ to upgrade your software and then we'll talk to you.' I can tell you from experience (as we had 5 Windows 7 圆4 computer systems at our shop running Flexi 8.1 for three months without install errors, crashes, or real issues, until we did decide to upgraded software) that Flexi version 8.1 WILL work on Windows Vista and Windows 7.